The present invention relates to a process for making electroinsulation enamels which can be diluted with water.
Electroinsulation lacquers are in wide use as wire enamels for insulating electrical conductors and for instance as immersion lacquers for impregnating coiled wires.
The requirements of the electrical industry regarding heat resistance of the insulating materials have increased consistently during the last few decades. The requirements are particularly high regarding insulation of electrical conductors. The reason is the increasing tendency for miniaturizing of electrical appliances, improving the weight-performance relation in case of electrical machines and the ensueing temperature increase in the coils. Of moment is also that electrical machines are frequently used in plant installations where the atmospheric conditions already involve high temperatures.
As far as can be predicted future increases of the heat resistance will be required in case of wire enamels. A wire enamel type which meets to a large extent the requirements of the electrical industry and in addition has excellent mechanical and electrical properties, is constituted by wires coated with polyesterimide lacquers. These polyesterimide wire lacquers and insulated wires now have been part of the art for more than 10 years; see German published applications No. 1,445,263 and 1,495,100 corresponding to U.S. Pat. No. 3,697,471.
The polyesterimide resin base of these lacquers is made preferably of aromatic tricarboxylic acid monoanhydrides, aromatic dicarboxylic acids or lower dialkylesters thereof and diamines. Preferred among these are aromatic diamines. A further element of the esters are di- and higher functional alcohols.
The solvents for the polyesterimide wire enamels are almost exclusively phenolic materials, preferably isomeric cresol and xylenol mixtures obtained from coal- or lignite tar and blended with aromatic hydrocarbons such as solvent naphtha.
The contents of solvents in the polyesterimide wire enamels usually is between 60 and 75% of which about 65% are cresols and xylenols.
A decided disadvantage in using wire lacquers with such high percentages of solvent contents is the problem of the air pollution by these solvents which have a highly obnoxious character.
The law in general now requires that the air pollution by discharge gases be reduced to a minimum and for this purpose expensive steps are necessary for the lacquer application such as various catalysts.
It is furthermore necessary to mention also the highly dangerous character of the cresolic solvents when coming in contact with the skin and of the aromates which are used as blending components in case of inhalation of the vapors. It is particularly this inhalation which often cannot be avoided in industrial practice.
All this shows that a process for insulating electrical conductors with wire enamels which, while preserving the high performance level of the prior art lacquers, do not contain this type of solvents would therefore constitute a definite and urgent contribution to the state of the art.
Apart from the coating of electrical conductors with solvent-free resins from a melt which has already been proposed the use of water as solvent and diluent for polyesterimide resins would be ideal since the apparatus for applying the enamels and various coating appliances could be used without any necessity for structural changes.
This would also solve the problems in regard to air pollution and in regard to the other dangers caused directly by the solvents as discussed above.
The making of water soluble polyesters has already become known from various prior art disclosures. In these cases there is used as excess of carboxylic groups, for instance by concurrent use of tricarboxylic acids, and the polyesters which then contain free carboxylic groups are thereafter neutralized with alkali, that is they are converted to the water soluble salts.
In German application No. 17 20 321 corresponding to British Pat. No. 1,184,139 a process is described for making polyesterimide resins which can be diluted by water. According to that disclosure aromatic tricarboxylic acid anhydrides are first reacted with up to 80% of the stoichiometric amount of primary diamines necessary for the imide formation and are then condensed with an excess of di- or trialcohols. The excess alcohol is then distilled off and the condensation product is heated with a small amount of aqueous ammonia and possibly upon addition of dialcohols to a temperature in excess of 80.degree. C and is subsequently diluted with water. It has, however, been found that the thus-obtained aqueous polyesterimide solution when applied as lacquer to a wire does not maintain the required level of properties as is presently obtained with the conventional polyesterimide wire lacquers.